The Center for Neuroanatomy with Neurotropic Viruses (CNNV) was funded in 2004 to address the increasing needs of the research community for access to the use of viral transneuronal tracing tracers. This approach is the most widely used method to gain a circuit perspective on the functional architecture of the nervous system at a cellular level. The CNNV has become an integral resource for investigators by providing well-characterized reagents, guidance in the use of the method and translational support for successful grant applications. This application for continued funding of the CNNV is founded on the same goals and rationale advanced in the original submission. Specifically we propose to maintain a state-of-the- art National Resource that a) serves as a technical and intellectual resource for those interested in using the method, b) develops improved transneuronal tracing tools that permit more targeted dissection of the functional architecture of neural circuits, c) serves as a repository for well-characterized reagents essential to application of the method, and d) supports and stimulates the research enterprise of the NIH and other research agencies by providing sophisticated tools that enhance and expand the capabilities of individual investigators. The belief that this can best be achieved through the CNNV Is based upon the following assertions. First, by providing well-characterized reagents and essential expertise, the CNNV substantially reduces barriers that investigators experience in seeking access to the use of viruses as transneuronal tracers. Second, the unique combination of multidisciplinary expertise available through the CNNV provides the focus and platform for more rapid, targeted and Informed technology development. Third, the existence of unique resources and facilities within core laboratories of the CNNV greatly expands access to the technology. In this application we provide objective evidence supporting each of these assertions and demonstrate that the CNNV has rapidly evolved into a critical National Resource for the neuroscience research community. We also highlight the accomplishments of the Center and provide a plan for further improving the service and technology functions of the CNNV.